Presently there is a desire in the industry for higher circuit density in microelectronic devices which are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. There is a goal in industry to reduce feature size to 0.25 microns. The use of shorter wavelength radiation (e.g. deep UV e.g. 190 to 315 nm) than the currently employed mid-UV spectral range (e.g. 350 nm to 450 nm) offers the potential for this higher resolution. However, with deep UV radiation, fewer photons are transferred for the same energy dose and higher exposure doses are required to achieve the same desired photochemical response. Further, current lithographic tools have greatly attenuated output in the deep UV spectral region.
In order to improve resolution, Deckman et al. U.S. Pat. No. 4,608,281 (issued Aug. 26, 1986discloses the use of an electron beam exposure tool. Poly(methyl methacrylate) (PMMA) which undergoes main chain scission upon e-beam exposure is used as the resist material. After exposure the degraded polymer is removed with solvent to develop the image. Deckman teaches pre-exposure baking of the polymeric resist above its glass transition temperature (Tg) to remove solvent and improve resolution. Brault et al. U.S. Pat. No. 4,777,119 (issued Oct. 11, 1988also discloses pre-exposure baking of PMMA resist above its Tg to crosslink the polymer to improve its lithographic performance. Unfortunately, the lithographic mechanism of Deckman and Brault of main chain scission polymer degradation requires high exposure doses of radiation and is not suitable for manufacturing processes.
In order to improve the sensitivity of a resist for use in the deep UV, Ito et al. developed an acid catalyzed chemically amplified resist which is disclosed in U.S. Pat. No. 4,491,628 (Jan. 1, 1985). The resist comprises a photosensitive acid generator and an acid sensitive polymer. The polymer comprises side chain (pendant) groups which are bonded to the polymer backbone and are reactive towards a proton. Upon imagewise exposure to radiation, the photoacid generator produces a proton. The resist film is heated and, the proton causes catalytic cleavage of the pendant group from the polymer backbone. The proton is not consumed in the cleavage reaction and catalyzes additional cleavage reactions thereby chemically amplifying the photochemical response of the resist. The cleaved polymer is soluble in polar developers such as alcohol and aqueous base while the unexposed polymer is soluble in nonpolar organic solvents such as anisole. Thus the resist can produce positive or negative images of the mask depending of the selection of the developer solvent.
Nalamasu et al., "An Overview of Resist Processing for Deep-UV Lithography", J. Photopolym. Sci. Technol. 4, 299 (1991) also discloses a chemically amplified resist composition comprising a photoacid generator and poly(t-butoxycarbonyloxystyrene sulfone).
Schlegel et al., "Determination of Acid Diffusion in Chemical Amplification Positive Deep Ultraviolet Resist", J. Vac. Sci. Technol. 278 March/April 1991 discloses a chemically amplified resist comprising a photoacid generator and a chemically amplified dissolution inhibitor p-tetrahydropyranyl protected polyvinylphenol disposed in novolac resin. Schlegel teaches using a high pre-exposure bake temperature in combination with a low post-exposure bake temperature. However, due to the high absorbance of the novolac resin in the deep UV, such a composition is unsuitable for use in semiconductor manufacturing in the deep UV.
Further, because of the catalytic nature of the imaging mechanisms, these chemically amplified resist systems are sensitive toward minute amounts of airborne chemical contaminants such as basic organic substances. These substances degrade the resulting developed image in the resist film and cause a loss of the linewidth control of the developed image. This problem is exaggerated in a manufacturing process where there is an extended and variable period of time between applying the film to the substrate and development of the image. In order to protect the resist from such airborne contaminants, the air surrounding the coated film is carefully filtered to remove such substances. Alternatively, the resist film is overcoated with a protective polymer layer. However, these are cumbersome processes. There still is a need in the art for a process for imaging chemically amplified resists for use in semiconductor manufacturing.
It is therefore an object of the present invention to provide an improved process for imaging of photoresist.
Other objects and advantages will become apparent from the following disclosure.